Embodiments of the present invention relate generally to electromagnetic-signal receivers, and more particularly to receivers implementing weighted signal equalization.
Transmitter diversity is a known technique for reducing the effects of fading in a wireless communications system. Transmitter diversity can be used for chip equalization, and especially to maintain orthogonality between spreading codes in systems such as the WCDMA (Wideband Code-division Multiple Access) system.
A wireless communication channel in which the signal reflects off different objects in its path before reaching the receiver is said to exhibit multipath propagation. “Fading” is a consequence of multipath propagation. As the various reflected signals take different paths, they may arrive at the receiver at different times and can combine to cause destructive interference, resulting in signal fading at the receiver or causing interference between signals that were intended to be mutually orthogonal.
As is known to those ordinarily skilled in the relevant arts, “transmitter diversity” was developed in order to compensate for effects such as fading and interference. In a wireless communication system featuring transmitter diversity, one station (for instance a base station) is equipped with two or more antennas, all of which are used to transmit a signal to a particular other station (for instance a mobile station). The signal transmitted from each antenna is the same. However, the signal is transmitted from a second antenna with a phase delay relative to its transmission from a first antenna. The phase delay is chosen with the aim that the signal from the second antenna (“the second signal”) arrive at the mobile station correctly in phase with the signal from the first antenna (“the first signal”), causing constructive interference, and hence mitigating any fading otherwise experienced at the mobile station.
The signal paths may be regarded as individual channels. In order to calculate how much phase offset must be applied to the second signal, the transmitting system requires access to the transfer function for both signal paths (channels). Such transfer functions are alternatively referred to as channel estimates. The channel estimates for the channel between the first antenna and the mobile station (channel estimate H1) and for the channel between the second antenna and the mobile station (channel estimate H2) are calculated. The channel estimates are then used to calculate the required phase compensation for the second antenna in order to reduce signal fading or interference, and thereby increase the signal to noise ratio at the receiver.
When more than one mobile station is configured to receive a particular signal from the base station, a problem arises relative to a system such as that described above. More specifically, when the base station is configured to correctly phase compensate the signal transmitted for a particular mobile station in the manner described, the signals received by the other mobile stations are unlikely to be correctly phase compensated. Instead, the signal received by other mobile stations will be incorrectly phase compensated and may result in an even lower quality signal than that of a system that does not employ transmitter diversity.
According, there exists a need for a system and method for improving phase compensation relative to signal systems in which a base station transmits signals to multiple mobile stations.